This invention relates in general to heat transfer apparatus and methods for evaporating, distilling, freezing, heating or cooling liquids, and more specifically, to an orbital drive for a whip rod used in conjunction with a tube and surrounding shell type of heat exchanger.
When processing fluids, it is often required to transfer heat to or from the liquid using a heat exchange surface, typically one formed of sheet metal, and a second process fluid on the opposite side of the sheet metal that is at a different temperature than the liquid being processed. This heat transfer between fluids may serve to warm the process fluid or cool it, as in a glycol chiller commonly used in building air conditioning systems. It may also serve to change the phase of the fluid, as in the production of fresh water by boiling it from sea water, or the production of ice slurries by partially freezing water or a water solution. Ice slurries are useful, among other applications, for cold storage to reduce peak load power demands in building air conditioning systems and to provide refrigeration for food such as milk stored on a dairy farm for transport to a processing plant and fish catches stored on fishing vessels.
The size, and hence cost, of a heat exchanger depends on the heat transfer coefficient, which reflects resistance to heat flow through a layer of a "hot" fluid, a heat exchanger wall separating the hot and cold fluids, a layer of a "cold" fluid, plus deposits forming on either hot or cold surfaces of the wall. For economic reasons, a substantial temperature gradient is required to drive the heat transfer through these resistances. This high gradient limits the energy efficiency of evaporators or freezers by either limiting the number of stages or imposing a higher lift on a vapor compressor.
U.S. Pat. No. 4,230,529 and 4,441,963 issued to one of the present applicants disclose a new approach to solving these problems. They involve using a vertical, thin-walled, open-ended heat transfer tube (or tubes) driven in an orbital or wobbling motion. This orbital tube motion increases the heat transfer efficiency by reducing the thermal resistance at the inner and outer surfaces of the tube. The motion swirls a liquid to be evaporated into a generally thin film over the inner surface of the tube. This increases the evaporation surface area and decreases the thermal resistance by decreasing the thickness of the liquid layer. The orbital motion also aids in heat transfer into the tube at its outer surface produced by condensation of a heated vapor stream. The condensation increases the thickness of the liquid layer at the outer surface, and hence its thermal resistance. The orbital motion throws off the droplets, thereby increasing the heat transfer at the outer wall.
Both of these patents teach multiple such tubes held in a common container. Eccentrics drive the tubes to undergo a wobbling motion in a horizontal plane. The liquid is driven in turn by a dynamic coupling to revolve over the inner surface as it flows down the tube under the influence of gravity. These arrangements require cranks, bearings and complicated seals inside the evaporator that accommodate this movement. The component parts are difficult and costly to manufacture and assemble, they must be machined to close tolerances, they are susceptible to corrosion and contamination when used in the chemical industry, and they wear, which leads to a deterioration in the balance of the wobbling tubes and attendant vibrations. The '529 patent also discloses a self balancing arrangement with a self adjusting orbital radius that accommodates the balance to changes in mass. However, if the base moves, e.g., if the apparatus is mounted on a moving reference frame such as a ship at sea, the crank radius must be fixed, and even this step may not be adequate.
Many known heat transfer devices ranging from ice cream makers to sophisticated evaporators use a rigid wiper bar that is positively driven to rotate within the tube to spread viscous liquids into a thin, evenly distributed film. Positively driven wipers can handle fluids with a viscosity of 1,000,000 c.p. or higher. (Water has a viscosity of 1 c.p.) However, known heat transfer devices using rigid, positively driven wiper or scraper have drawbacks. First there is a need to introduce into the evaporator or freezer, and to seal, a rotational drive shaft. Second, because the wiper or scraper is rigid and moving over a fixed surface at close spacings, manufacturing and assembly become difficult and costly. The surface must be machined to close tolerances, as well as the wipe/scraper and its support structures. Further, these known rigid wiper arrangements are susceptible to, and comparatively intolerant of, wear.
To solve these problems for less viscous fluids, e.g. those with a viscosity of 1 to 1,000 c.p., U.S. Pat. No. 4,618,399 describes a whip rod located in the tube which spreads the feed liquid into a highly then and uniform film to reduce its thermal resistance and to enhance its evaporation. The whip rod also controls the build up of solid residue of evaporation. The '399 patent discloses several arrangements for mounting the rod, including lengths of cables, a flexible, but non-rotating anchor connected between a base and the lower end of the rod, and a double universal joint also connected between the lower end of the whip rod and the base. While the whip rod is effective as a film distributor, the mounting arrangements have disadvantages. They increase the overall material, assembly and operating costs. Also, they fail. Material fatigue of flexible cables supporting the whip rods is a particular concern.
U.S. Pat. No. 4,762,592 describes an orbital drive that overcomes the manufacture, assembly, wear and balance problems of the earlier eccentric-crank drives. This improved drive uses a rotating counterweight or weights mounted on the evaporator and a spring-loaded strut suspension for the evaporator. The counterweights and the mass of the evaporator revolve around one another as the counterweights rotate.
While this arrangement does overcome the problems associated with an eccentric crank drive, it also suffers from certain deficiencies. For example, it requires the orbital movement of a large mass, particularly where the unit is scaled up to a commercial size with multiple large tubes, each carrying a liquid stream. This mass increases the power requirements (particularly on start up), increases the demands on the spring-strut suspension, can lead to an early fatigue failure of the suspension, and generally increases the construction and operation cost of the system. It also increases the desirability of a stable operating platform, e.g. a concrete floor, as opposed to one that moves such as a ship at sea or some other transport. While the '592 patent proposes a solution to the moving platform problem, the solution in practice has not been adequate when the apparatus has been scaled up to commercially useful sizes. One problem was that when the unit was scaled to a commercially acceptable size, motion of the base placed unacceptably high loads on a crank or cranks that drove the entire unit into an orbital motion.
While the orbital tube approach has been used for evaporation and distillation, in the prior art it has not been applied for freezing. One reason is that the liquid freezes to the heat transfer surface, increases the resistance to a heat flow through the exchanger, and thereby greatly reduces any performance advantages of the orbital tube approach.
Currently, there are two principal types of cold storage systems on the market using ice. One is known as the ice harvester type, where a group of ice making machines are installed over an open storage tank. Ice grows to a certain thickness before being periodically harvested into the tank by a defrosting cycle. The other one is known as the ice bank type. It employs a group of low cost heat transfer units, usually made of plastic, on which all the ice needed for cold storage accumulates continuously during each chilling cycle. In either of these two types, the effectiveness of transferring the heat from the water to the refrigerant during the ice forming process is not as efficient as desired, thus increasing equipment cost.
The concept of making ice in slurry form so that the ice making machine can operate continuously, with interruption, and with some improved heat transfer property has been attempted in the industry by companies such as the Chicago Bridge and Iron, Inc. and more recently, by the Electro Power Research Institute ("EPRI") with their scheme publicized under the rade designation "Slippery Ice". At the present time the performance of the Slippery Ice cold storage system is believed to be in the evaluation stage.
The EPRI sponsored research to develop a "Slippery Ice" system was reported in an article entitled "Cool Storage: Saving Money and Energy" published in the July/August 1992 issue of EPRI Journal. In the EPRI scheme, calcium magnesium acetate is added to the water. According to EPRI, the use of this additive causes ice to form in the liquid pool, away from the heat exchanger surface, and results in a slushy type of substance that does not cling to metal. The advantages of the "Slipper Ice" for improving the economy were also reported in Sep. 27, 1992 edition of The New York Times entitled "Keeping Buildings Cool With Greater Efficiency". In this article the use of automobile antifreeze in the water to be frozen was reported to be unsatisfactory because it tends to lower the freezing point to much.
The Slippery-Ice concept is attractive because it causes an ice slurry to flow down a chilling surface under the influence of gravity only, without mechanical aid. While Slipper Ice works, how it works is not known. Moreover, this approach has several significant drawbacks. First, only one known additive lets ice overcome the initial stickiness barrier to a gravity feed of crystals down the chilling surface. This is of particular concern where the liquid being processed in a food product; this additive cannot be used. Another limitation is that the heat flux, wetting rate and additive concentration must be carefully controlled for the Slippery Ice to form. Also, the heat transfer surface must be electropolished.
In many circumstances, it is undesirable to require that the liquid pass through the heat exchanger in a falling film. If the heat exchanger tubes were flooded, then the liquid supply pressure would be sufficient to transfer it to the next processing step. The complication and expense of an additional pump and level control system, which are usually required in systems using a falling film heat exchanger, would not be necessary. Heretofore orbital heat exchangers have been limited to operation with a falling film. One reason is that because the entire apparatus orbits, or the tubes within the outer shell orbit, flooding the tubes greatly increases the mass being orbited. This in turn increases the power required for operation, increases wear, and increases vibration/balance problems. In addition, movement of whip rods within flooded tubes is, in general, impeded by the liquid, or moves in coordination with the revolving body of liquid in the tube so that the rod has a diminished effect on the heat transfer process.
In certain applications it may be desirable to orient the heat exchanger tubes other than vertically. For example, on a ship, the pitch and roll of the ship with the waves would require expensive gimbal arrangements to operate with the tubes in a vertical orientation. Even in land-based systems, non-vertical orientations may be desirable to accommodate restrictions on equipment height, to fit through doors or under existing ceilings, and to ship units in standard shipping containers.
It is also desirable to reduce the vibration produced by unbalanced rotating or revolving masses (e.g. whip rods and drive members). The vibration is easily seen by the user and raises concerns about equipment durability and possible fatigue failure of pipe connections to the heat exchanger. It also increases the power consumed and places an increased stress on the mounting arrangement. Heretofore, one solution has been to orbit tubes in groups with a 180.degree. phase different between the groups. The '529 and '963 patents illustrate this approach. The '592 patent discloses orbiting counterweights.
Recent development work suggests that as the orbital heat exchange units are scaled up to more commercially useful sizes and operated under conditions that maximize the heat transfer flux, a new set of design problems come to the forefront. Using orbital technology, a straightforward way to scale up at the desired surface-to-volume ratio is to use more tubes. For example, a twenty ton freezer/chiller can have forty-two 11/2 inch diameter tubes. A thirty ton evaporator of the vapor compression type for sea water desalinization can have two hundred fifty-eight tubes. With this many tubes, significant problems occur in delivering the required torque to all the tubes with the correct phase relationship, providing manageable wear, keeping vibration low, part count low, and providing assembly ease become roadblocks to commercially-sized multi-tube apparatus.
Known orbital drives and conventional wiper arrangements do not meet demands of such multiple tube set-ups. The masses of the tube, or containers and tubes, place extreme strains on rotary bearings of eccentrics coupled between a rotary power source and the end application of the force. Large forces quickly produce wear in bearings and at drive surfaces causing play in the drive train and a loss of the desired phase relationships between the movement of groups of tubes. Large forces also increase the friction in the drive train. To crank rods at each of many tubes using an array of gears or pulleys, even without considering wear, presents a daunting mechanical design problem, particularly if this mechanical array is exposed to, or must be compatible with a feed fluid. The wear problems, even in known wiper systems such as the freezer/chiller sold by Sunwell using rotating wiper blades acting on a heat exchange surface requires initial precise tolerancing and thereafter recommended annual re-conditioning of the blade assembly at a cost in excess of many thousands of dollars per re-conditioning. In practice, wear not only affects performance, but also can have a dramatic impact on the ongoing cost of operation.
A special concern in ice-slurry applications is that at a high cooling rate, ice forming on the heat transfer tube can not only reduce heat transfer efficiency, but it can also grow to fill the tube with ice and eventually freeze the whip rod in the center of the tube. Also, there is typically more ice near the bottom of the tube than the top. It is therefore desirable to reduce the number and size of mechanical obstructions to the exit of the ice slurry from the bottom of the tubes. This suggests a top mounting of the rods for this application. A whip rod orbiting within the tube, however, may nevertheless experience a variation in the mechanical resistance to its movement due to variations in the amount of ice present in the tube as a function of its length. This is more likely to occur as the exchanger is operated at a high heat flux. As a result, the whip rod may not "center", that is, align itself with the vertical axis of the tube, but rather it may assume a skewed, or cocked orientation characterized by its lower end trailing its upper end. Any such skewing is highly undesirable since it interferes with the beneficial action of the rod when it is fully engaged with the tube.
In addition, while various arrangements have been tried to lower the flux resistance at the outside of the tube due to condensation, these techniques are not useful where evaporation or boiling occurs at the outside surface. Clearly the energy efficiency of freezing and chilling applications can be enhanced by decreasing thermal resistance at the outer tube surface. No known apparatus or techniques accomplish this end.
It is therefore a principal object of this invention to provide an orbital heat exchanger, a thermal storage system using that exchanger, and a process of heat exchange that can operate either in a flooded or falling film mode, and either in a vertical or non-vertical orientation, or on a fixed or moving base.
Another principal object is to provide an orbital drive for a rod-in-tube type orbital heat exchanger that can be readily scaled up to drive multiple tubes with positive tangential and radial components of the drive force on the rod to deal with high viscosity liquids and solid deposits on the tube wall.
Still another principal object is to provide an apparatus and method for enhancing the heat transfer at the outside of a tube used for freezing or chilling.
A further object is to provide an orbital heat exchanger with an orbital drive that has a comparatively low mass and a comparatively low power consumption.
Another principal object is to provide an orbital drive for a rod-in-tube type heat exchanger where the rod is self-adjusting to maintain a parallel alignment with respect to the tube.
A further object is to provide an orbital drive with the foregoing advantages while being substantially insensitive to wear or drive parts and having no critical tolerances.
A further object of the invention is to provide a heat exchanger and orbital drive which can operate as a freezer/chiller at a high heat flux of the exchanger.
Still another object of the invention is to reduce the amount of vibration produced by an orbital heat exchanger.
Yet another object is to provide an orbital heat exchanger with the foregoing advantages which has a favorable cost of manufacture both in terms of a low part count, in terms of ease of assembly for a low manufacturing cost and easy disassembly for low cost maintenance.